This invention relates to a Stirling engine and particularly to the type known as double-acting in which each piston acts as a power piston and a displacer simultaneously. Heat from an outside heat source such as an external flame is added to the engine through a heater head assembly containing a working gas. The working gas is expanded to operate a piston within a working chamber; continuous external heating and cooling of the working gas provides for a full cycle engine. The complete cycle takes place in one revolution of the crankshaft as opposed to multiple revolutions required by conventional piston engines. To make the engine more practical, regenerators are located between the fixed heating and the cooling sources; such regenerators store otherwise wasted heat during the cooling process and permit recovery of the heat during the heating phase. This stored heat is equal to several times the heat added from the outside heat source.
One of the most difficult problems for the Stirling engine is to optimize heat transfer characteristics through the walls of the heater tubes or pipes, since the output of the engine is dependent thereon. It is desirable that the heat absorbing surface area of the heater head assembly or tube complex should be as great as possible and that the heat flux be as great as possible. However, the volume of the gas in the tubes or pipes must be as small as possible as this volume is a "dead" volume in the working cycle; similarly it is desired that the resistance against the flow of the working gas through the heater tubes should be as low as possible.
Heretofore, prior art constructions have attempted to meet such conflicting goals by utilizing tubes of a very small internal bore and using such tubes in great numbers to permit a large surface area to come in contact with the working gas passing within. The tubes were arranged to cause the working gas to flow on a singular path from the working chamber to or from the regenerator. In most instances, such tubes were exposed to one single pass of the surrounding heating medium and in certain instances, as described more fully in the detailed description, a partial double pass was provided. With these constructions, the temperature of the exhaust gases, having passed through and against such heater tubes, was at an extremely high temperature range indicating that the heat content thereof was not transferred as efficiently as possible to the working gas within the tubes.
To meet this problem, regenerative wheels have been utilized so that heat could be collected from the exhaust gases and returned to the incoming air to be combined with fuel for combustion; thus the exhaust heat content was in part returned to the cycle. Such regenerative wheels present many attendant problems including unnecessary cost.
In another attempt to meet this problem, the heat tubes were arranged not only to project linearly from the working chamber but were additionally bent to have a U-shape and/or connected to a ring-shaped manifold located above the working chamber and regenerator. These arrangements have not allowed an increase of the number of pipes due to space. Accordingly, it was necessary to increase the length of the pipes and thus also to increase the resistance against the gas flow therein. Fins or heat conducting surfaces were added to the lengthened portions of the pipes to improve heat transfer; such fins have proved to be costly, fragile and difficult to manufacture. It has become evident that this manner of increasing heat absorbing capacity is limited and in certain respects undesirable.